Extension of characterization of coagulation factors of lamprey blood, one of the most primitive vertebrates, was performed. The subunits of lamprey fibrinogen, especially the (A)alpha chain, was examined to elucidate its structure. Salmon fibrinogen and fibrin were examined to determine the molecular weights of the (A)alpha, (B)beta, and gamma subunits. Laser light scattering, a non-invasive technique for measuring rigidity of soft gels, was utilized as a quantitative procedure to correlate the change of rigidity of non-crossed linked and cross-linked fibrin clots.